TY - JOUR
T1 - Critical behavior within 20 fs drives the out-of-equilibrium laser-induced magnetic phase transition in nickel
JF - Science Advances
JO - Sci Adv
DO - 10.1126/sciadv.aap9744
VL - 4
IS - 3
SP - eaap9744
AU - Tengdin, Phoebe
AU - You, Wenjing
AU - Chen, Cong
AU - Shi, Xun
AU - Zusin, Dmitriy
AU - Zhang, Yingchao
AU - Gentry, Christian
AU - Blonsky, Adam
AU - Keller, Mark
AU - Oppeneer, Peter M.
AU - Kapteyn, Henry C.
AU - Tao, Zhensheng
AU - Murnane, Margaret M.
Y1 - 2018/03/01
UR - http://advances.sciencemag.org/content/4/3/eaap9744.abstract
N2 - It has long been known that ferromagnets undergo a phase transition from ferromagnetic to paramagnetic at the Curie temperature, associated with critical phenomena such as a divergence in the heat capacity. A ferromagnet can also be transiently demagnetized by heating it with an ultrafast laser pulse. However, to date, the connection between out-of-equilibrium and equilibrium phase transitions, or how fast the out-of-equilibrium phase transitions can proceed, was not known. By combining time- and angle-resolved photoemission with time-resolved transverse magneto-optical Kerr spectroscopies, we show that the same critical behavior also governs the ultrafast magnetic phase transition in nickel. This is evidenced by several observations. First, we observe a divergence of the transient heat capacity of the electron spin system preceding material demagnetization. Second, when the electron temperature is transiently driven above the Curie temperature, we observe an extremely rapid change in the material response: The spin system absorbs sufficient energy within the first 20 fs to subsequently proceed through the phase transition, whereas demagnetization and the collapse of the exchange splitting occur on much longer, fluence-independent time scales of ~176 fs. Third, we find that the transient electron temperature alone dictates the magnetic response. Our results are important because they connect the out-of-equilibrium material behavior to the strongly coupled equilibrium behavior and uncover a new time scale in the process of ultrafast demagnetization.
ER -